Well, if you want to get down to science, our biology is supposed to pair us off, at least for 2-3 years. We are not necessarily supposed to be monogamous, but we are for at least that period of time.
I'm sure of a couple of you can correctly guess as to why we are supposed to fall in love for at least 2-3 years, especially if you've read another thread I made or just read some of my posts on the topic. We are supposed to fall in love (which, is a nice "chemical" romance, as well.), mate, and stay in love for about 2 years. Shortly after the 2 year mark, the "chemical romance" starts to wear off and the relationship, in order to continue, is more up to the personalities and actions of the individuals. No longer can the couple rely on that "high" from the initial "in love" portion of the relationship. They have to work at it. Their behaviors may not change, but the motivations behind those behaviors wil.
Of course, that's just an ideal situation. Humans are such complex social creatures that we can easily go against the grain of our "chemical romance." In other words, we can f*** up a good thing by doing something stupid.
To split it up among the genders (not modern, but hunter and gatherer):
Females would prefer that the males stick around, even after the initial phase of biology runs its course. Stick around to provide for the family.
Males, especially successful ones (ones with high testosterone levels, large in stature, intelligent, etc(, will want to copulate with other females. Sure, he may settle down for a few months to a couple of years, but he will want to move on.
The females will try to keep the male as the male provides security and food. The female also desires this for her young.
Now, back to the 2-3 years item I spoke of earlier. Why does that chemical phase last that long? To ensure that there are two parents to help raise the child through the most treacherous part of their life. The first 2 years of early human's life saw the highest mortality rates. Obviously, evolution took the proper course by having those that stuck together long enough for the child to "make it over the hump." No doubt that had we not been so intelligent, we may have continued to breed that into our future species success. We could have become like parrots where the vast majority bond for life and remain close throughout that life. However, due to the intelligence that evolved, humans also become very socially complex and that really messed with just about everything that our biology tells us to do.
So, where is all of this going? Obviously, marriage.
Marriage or "life-bonds" have been around since early man. This probably arose from couples bonding beyond that 2-3 year period. Being in love is not just a chemical thing, even in the first 2-3 years. Really, we can say that marriage; or rather, pairing off; was bred into our species success. It served a function in the preservation/propogation of the species.
Now, some have suggested that marriage is archaic or antiquated. Maybe...but not really. Sure, the ceremony/ritual of marriage may be old, but the bonding of two individuals is not old. That's part of our biology.